FYI, the licence on these is copyleft. I don't think it is legally clear whether an entire app or website would take on the licence, and I would be surprised if Google tried to enforce the licence in that way, but it's worth keeping in mind in case using it for anything particularly critical.

https://github.com/google/material-design-icons/blob/master/...

http://creativecommons.org/licenses/by-sa/4.0/